Overcenter mechanism

ABSTRACT

An overcenter mechanism in which the work required to move the mechanism overcenter from the respective sides can be made asymmetrical. For example, while the force needed to move the mechanism in a forward direction overcenter remains substantial, that required to return it to the center position can be made zero or some other chosen value different from, and preferably smaller than, the forward force. This effect is achieved by building up a specially shaped surface on one or more of the link members of the mechanism and causing this surface to roll on a flat surface as the mechanism moves from side to side across the center position. Alternatively, the flat surface can be built up with a ramp so as itself to be curved. One useful application of the mechanism is to avoid the need for special release mechanisms in locking wrenches of the overcenter type which otherwise can be very difficult to open.

United States Patent Inventor Harry T. Stevinson Ottawa, Ontario, Canada Appl. No. 671,363

Filed Sept. 28, 1967 Patented Apr. 13, 1971 Assignee Canadian Patents and Development Limited Ottawa, Ontario, Canada OVER-CENTER MECHANISM 11 Claims, 11 Drawing Figs.

Primary Examiner-Robert C. Riordon Assistant ExaminerRoscoe V. Parker, Jr. AttorneyStevens, Davis, Miller & Mosher ABSTRACT: An overcenter mechanism in which the work required to move the mechanism overcenter from the respective sides can be made asymmetrical. For example, while the force needed to move the mechanism in a forward direction overcenter remains substantial, that required to return it to the center position can be made zero or some other chosen value different from, and preferably smaller than, the forward force.

This effect is achieved by building up a specially shaped surface on one or more of the link members of the mechanism and causing this surface to roll on a flat surface as the mechanism moves from side to side across the center position. Alternatively, the flat surface can be built up with a ramp so as itself to be curved.

One useful application of the mechanism is to avoid the need for special release mechanisms in locking wrenches of the overcenter type which otherwise can be very difficult to open.

II/IllI/II/l ,ovanennaitaamaatsn This invention relates to improvements in an overcenter mechanism'of the type consisting essentially of a pair of link members that are pivotally connected together at adjacent ends, such link members having remote ends that are respectively engaged by external members which may take the form of a pair of parts of a device that are relatively movable towards and away from each other, e.g. parts of a circuit breaker, or of a locking device, or of some other mechanism that it is desired to retain in a set position.

When the overcenter mechanism is in its central condition, that is, when its link members are in line with one another, a very small transverse force acting on the joint interconnecting the members can move such joint to one side or the other. The mechanism will be stable in either of these side positions, since, in order to move it back past the central position, it will be necessary to do work to spread apart the remote ends of the two members, together with the other device (external) members to which they are connected. It is assumed that there will be a spring or other force tending to urge the two remote ends of the link members together, with the result that a force has to be exerted to move them apart while moving the mechanism up to the central condition from either side. It is also assumed that the mechanism will be prevented by a stop or similar device from moving too far beyond the central position in either direction.

In certain applications of such overcenter mechanisms, while it is desirable to require that a significant amount of work must be done to force the mechanism overcenter in a forward direction, it is undesirable for as much work to be required to move the mechanism back overcenter. While it may be desirable to require that some effort be needed to make this return movement, in order to render the mechanism stable, it is found in some applications that the need to exert a return force as great as the initial operating force is disadvantageous. One such application, which will be referred to in more detail below, resides in the use of overcenter mechanisms in wrenches of the locking or so-called toggle joint type. Indeed, it has become commonplace to provide special release devices to overcome the return force problem in such wrenches.

The object of the present invention is to afford improvements in respect of such disadvantages by providing an overcenter mechanism that is asymmetrical in respect of the forces required to bring it up to its central position from the two sides thereof. As will be explained below, the force from one side may be reduced to zero (excluding that required to overcome friction), or it may be reduced (or even increased) to some other value different from that required to bring the mechanism up to center from the other side.

It will be appreciated that these forces do not remain constant in all positions of the mechanism, even in a conventional overcenter mechanism. They vary with the extent to which the mechanism deviates from the central position, and it is inherent in the nature of this type of mechanism that the deflecting force will reduce to zero as the central condition is approached. Thus, in comparing the deflecting forces needed to bring the mechanism to its central condition from the respective sides, and in saying that, in accordance with the present invention, these forces are made unequal to each other (or that the work needed to be done by these forces is unequal), it is to be understood that for such comparisons to be meaningful they must be made between comparable physical conditions of the mechanism on the two sides, i.e. under conditions in which the mechanism has the same deflection from the central condition on each side.

Since the present invention is directed towards improvements in such overcenter mechanisms themselves, it will first be described in connection therewith, and then subsequently in connection with one possible application, namely the locking type wrench.

The invention will become more apparent from the diagrammatic illustration thereof provided by way of example in the accompanying drawings, in which:

FIG. la shows an overcenter mechanism modified in accordance with the present invention, in a relaxed position;

' FIG. lb shows the same mechanism in the central position;

FIG. lc shows the same mechanism in an initial overcenter position;

FIG. ld shows the same mechanism in a further overcenter position;

FIG. 2a shows a modification of the mechanism of FIGS. la to d in its central position;

FIG. 2b shows the mechanism of FIG. 2a in an overcenter position;

FIG. 3 shows a locking wrench with the mechanism of FIGS. la to d applied thereto;

FIG. 4 is an enlarged fragment of FIG. 3;

FIG. 5 is a modification of FIG. 4; and

FIGS. 6 and 7 are respectively further modifications of the overcenter mechanism of FIGS. la to d, also shown in the central position.

FIG. Ila shows diagrammatically an overcenter mechanism of the type with which the invention is concerned, comprising a pair of link members I0 and 11, adjacent ends of which are pivotally connected together by a joint pin 12. The remote end of the first link member 10 is shown pivotally connected by a pin I7 to an external member III that represents a device (not shown) that is to be controlled by the overcenter mechanism. It is assumed that the external member 13 exerts a downward force on the overcenter mechanism, eg a spring force, as represented by the arrow A.

. In a conventional overcenter mechanism the remote end of the second link member 11 would typically be pivotally connected to a fixed base. However, in accordance with the present invention, the lower end of the link member 11 is extended laterally into the form of a shoe 14 that has a specially shaped cam surface 15 located for rolling on a flat face 16, which represents the other external member, relative to which the movement of the first external member 13 is to be controlled. To bring the overcenter mechanism from its relaxed position of FIG. 10 towards its central position of FIG. lb, it is necessary to exert a lateral force at the pin l2 in the direction of the arrow B. The force B must provide the work needed to move the member 13 upwards by the distance C against the force A.

If the member ll were conventional, in the sense of having its remote end simply connected to pivot at a fixed location on the lower surface I6, then, when the mechanism moved beyond the central position of FIG. lb, the member 13 would move downwardly again. As a result, any return movement of the mechanism back overcenter would require a lateral force E to act on the joint pin 12 in the direction opposite from that of the arrow B. However, by virtue of the specially shaped surface 15 of the shoe 14, as the mechanism moves first to the initial overcenter position shown in FIG. 1c, and then subsequently to the further overcenter position shown in FIG. la, the pin 17 stays at the same level as it occupied when the mechanism was in the central position of FIG. 1b. Any further movement of the mechanism beyond the FIG. 1d position is prevented by an adjustable stop screw 40. It follows then that the mechanism can now be returned from the position of FIG. Id (or of FIG. 1c to that of FIG. lb without any need to do work by pushing the member 13 out against the force A, and consequently without having to do any work other than overcome friction. In other words, the force E is zero.

In order to provide additional stability for the mechanism in its overcenter positions, the surface 15 may be so chosen that the pin l7 moves downwardly slightly during movement from the position of FIG. lb to that of FIG. lie or that of FIG. ld. This will require that some work be done in the return movement other than merely overcoming friction. The amount of such work becomes a matter of choice, but it is to be expected that the value chosen for the return force will be a minor proportion of the forward force. The prior art type of fixed pivot for the bottom end of the member ll involved a substantial dropping of the pin 17, e.g. comparable in magnitude to the dimension C, and this necessitated performance of too much work to return the parts overcenter to be convenient in some applications. The merit of the present invention is that this amount of work can be controlled to any extent desired, simply by selection of the specific shape of the surface 15.

Theoretically the surface 15 can be so shaped that the pin 17 will move upwardly slightly when the mechanism changes from the position of FIG. lb to that of FIG. 1c or that of FIG. 1d. However, such a device would tend to be unstable and would have a tendency to return automatically to a relaxed position such as that of FIG. la, and would therefore not normally have a useful practical application as an overcenter mechanism, except that in a case where a significant amount of friction was involved, a slight elevation of the pin 17 could be utilized to counteract part or all of this friction and thus yield a very sensitive device. At the other end of the scale, the surface 15 could be so shaped that the pin 17 fell more than an amount comparable to the dimension C, when moving to the FIG. 1d position. This would result in a mechanism that is harder, rather than easier, to return past the overcenter position. While this alternative is expected to be less attractive in practice, it serves to demonstrate the versatility of the present invention, and there may be some application in which it would be desired.

It is anticipated that, in practice, the main utility of the present inventive concept will lie in its application to situations, such as the one described below in connection with FIGS. 3 and 4, where it is desired to provide an overcenter mechanism that, while firmly stable in its overcenter position, nevertheless requires significantly less force to return it back past its central position than was originally required to move it in the forward direction up to the central position. Such an arrangement has the improvement advantage of avoiding need for critical adjustment of the stop screw 40.

The desired result can be achieved, as illustrated in FIGS. In to 1d, that is, with no change taking place in the position of the pin 17 on the overcenter side. This arrangement will apply to cases where friction alone is sufficient to achieve stability. Alternatively, and as already indicated, the surface 15 can be modified to permit a small downward change in the position of the pin 17, so as to require some work to be done for a return movement overcenter, thus enhancing stability. The main deciding factor in choice of the surface 15 will be the degree of stability required, measured against the ease with which the return overcenter movement must take place. It will be appreciated that there is infinite choice available in shaping the surface 15, including shapes that will permit nonlinear movement of the pin 17. For example, the surface 15 can be a compound shape, so as to produce a small drop in the level of the pin 17 in moving from FIG. lb to FIG. 1c, with no subsequent drop in moving from FIG. to FIG. 1d.

FIG. lb shows a broken line 18 which represents a true circular are described about the pin 12. It will thus be seen that the surface 15, in order to achieve the effect of FIGS. lb to 1d, involves the addition to the circular are 18 of a portion having a radial thickness D which increases from zero at the point 19 where the shoe 14 touches the surface 16 in the central position of FIG. lb to a maximum at the extreme righthand end of the shoe l4.

While the exact formula for D for a surface that will yield no change whatsoever to the level of the pin 17 is comparatively complex, a very close approximation to such an ideal surface is achieved, especially for angular displacements of the member 11 not exceeding about by fonning the surface 15 as an arc of a circle described about the pin 17. Hence, for simplicity, and in view of the fact that the present invention is essentially one in which practical considerations tend to prevail over the entirely theoretical, an important embodiment of the invention is one in which the surface 15 is a circular are described about the pin 17, i.e. a circular arc of radius equal to the combined effective lengths L of the members 10 and 11. Since obviously formation of a circular arc is easier than fomiation of some other more complex shape, there will normally be good reason for making the surface 15 circular. In cases where some falling of the pin 17 during movement beyond the overcenter position is specifically desired for increased stability, as discussed above, the surface 15 can be made as a circular arc of radius slightly less than the length L, say to percent ofL.

As indicated in FIGS. 2a and b, as an alternative, instead of adding the thickness D to the shoe, the latter can take the form of a shoe 14 with a surface 18 described as a circular are about the pin 12, i.e. with radius R1, the extra thickness D now being added to the face 16 in the form of a curved ramp surface 20. Again as an approximation, but nevertheless a remarkably close one, the desired value for D for no change in level of the pin 17 can be achieved by making the ramp surface 20 a circular are about the pin 17, i.e. of radius R1+R 2. This embodiment has the advantage that a force F acting along the member 10 will always pass through the point of contact 21" and will be normal to the tangents to both the circular arcs at this point, thus exerting no lateral force tending to make the surface 18 slip on the surface 20.

As yet another alternative, instead of all the additional thickness represented by D being added tothe shoe (as in FIGS. 10 to d) or all of such thickness being added on the platform (as in FIG. 2a and b), a combination of the two methods is possible (not shown) in which the total value of D is built up by the two surfaces combined.

As indicated above, while the thickness D shown is that which is theoretically desirable in order to achieve no change in the height of the pin 17 during movement beyond the central position, it may be that, in practice, a dimension equal perhaps to 80 percent or 90 percent of D will be chosen, in order to cause some dropping of the pin 17 on overcenter movement to enhance the stability of the mechanism, without adding sufficient resistance to the return motion to lose the advantage of the invention. In this way the return force for comparable displacements to each side of the central position would be from l0 percent to 20 percent of the forward force. It is thus further apparent that the present invention affords limitless choices in the selection of the shape of the surface 15 or the ramp 20 (in practice, selection of the radius of circular curvature of these surfaces) to achieve the desired degree of effort needed (and corresponding stability of the system) on returning the mechanism overcenter.

FIG. 3 shows a typical locking wrench in which the present invention is utilized. This wrench consists of jaws 22 and 23 pivoted together at a pin 24 under the control of handles 25 and 26 in the usual way. The handle 25 is pivoted to the jaw 22 by a pin 27 and also has a link 29 connected to it by a pin 28. When the jaws 22 and 23 are clamped about an object 30 by closing movement of the handle 25, the pin 28 moves overcenter in relation to a line extending from the axis of the pin 27 to the point of contact of a shoe 31 mounted on the end of the link 29 and engaging a flat end surface 32 of a conventional adjustable stop 33 (see also FIG. 4). By shaping the surface 34 of the shoe 31 in accordance with the teaching of FIGS. 10 to d (as shown in FIG. 4), or by shaping the surfaces 32 and 34 in accordance with the teaching of FIGS. 2a and b (not shown); or by some combination thereof, as already explained above, the force required to open the handle 25 and release the object 30 can be reduced to any desired percentage of that which is commonly experienced in conventional wrenches of this type. 4

FIG. 5 shows a modification of FIG. 4 in which the end surface 32' is inclined to make it normal to the centerline of the link 29 and thus reduce any tendency to slip.

The problem of opening this type of wrench has been a continuing one in the art. A mechanic is usually faced with no great difficult in closing the wrench with one hand, even in some relatively inaccessible location, because a squeezing motion is involved. But to reopen the wrench, especially if the location is such that he can only bring one hand to bear on the tool, is often very difficult, so much so that special release devices for forcing the mechanism back overcenter have been for example, U.S. Pat. Nos. 2,514,130 issued Jul. 4, l950 to H.

T. Jones and 2,589,511 issued Mar. 18, 1952 to C. H. Redmon. The present invention, in its application to tools of the locking wrench type, renders any such additional release devices unnecessary, because only very light pressure of the order of magnitude of that which can be exerted by spreading apart the fingers of one hand is required to open the jaws of the tool.

Further alternatives to the mechanisms of FIGS. 1 and 2 are shown in FIGS. 6 and 7.

FIG. 6 shows a pair of link members 40 and 41, pivotally connected together by a pin 42 and each having a shoe portion defining a circular surface 43 described about the pin 42. Both external members 44 and 45 here take the form of flat surfaces on which the circular surfaces 43 roll when the mechanism is moved overcenter by the transverse force B. As shown, with the surfaces 43 being circular arcs about the pin 42, the return force E needed will be theoretically zero. As in the other embodiments, this value can be adjusted above or below zero, as desired, by variation of the radius of curvature of the surfaces 43. It will also be apparent that the curvatures of the two surfaces 43 need not be equal to each other, just as the lengths of the two link members 40, 41 need not be equal I to each other, provided that the curvatures together produce the desired relative location of the external members 44, 45 under overcenter conditions.

FIG. 7 shows a reversal of FIG. 6, the outer or second ends of link members 50, 51 being pivoted by pins 52, 53 to external members 54, 55. The links have cooperating curved surfaces 56 that roll on each other and each is a circular ar described about the respective pivot pin.

'Throughout all the embodiments of the invention the various rolling surfaces may be roughened, or toothed to cooperate with rack teeth on the cooperating member surfaces, if this is found desirable to prevent slipping.

lclaim:

1. In an overcenter mechanism comprising:

a. first and second link members each having two ends;

b. interengaging means pivotally interengaging first ends of said link members together;

0. a pair of external members;

d. and means engaging the second ends of said link members each with a respective one of said external members;

e. said interengaging means being located between said external members for spreading of said external members apart, against a first force urging them together, by a second force acting on said link members in the vicinity of said interengaging means in a first direction transverse to said first force to urge said link members forwardly into a central position in which said interengaging means lies on a straight line between the points of engagement of said second ends with said external members, from a first lateral position in which said interengaging means is displaced to one side of said central position, or by a third force acting on said link members in a second direction opposite to said first direction to urge said link members rearwardly into said central position from a second lateral position displaced to the other side of said central position by an amount equal to the displacement of said first lateral position from said central position; the improvement wherein:

f. at least one end of one of said link members and the member portion engaged thereby comprise'a pair of cooperating surfaces so arranged that one surface rolls on the other surface as the mechanism is moved from either of said first. and second lateral positions to said central position; and

g. said surfaces being so shaped that said third force is different from said second force.

2. An overcenter mechanism according to claim I, wherein said surfaces are so shaped that said third force is less than said second force.

3. An overcenter mechanism according to claim 1, wherein said surfaces are so shaped that said third force is substantially zero.

4. An overcenter mechanism according to claim 1, wherein said surfaces are so shaped that said third force is a minor proportion of said second force.

5. An overcenter mechanism according to claim 1, wherein said surfaces are so shaped that said third force is not more than approximately 20 percent of said second force.

6. An overcenter mechanism according to claim 4, wherein said interengaging means for interengaging said first ends of the link members comprises a pivot pin, and wherein said cooperating surfaces are formed respectively on a second end of one of said link members, and on one of said external members.

7. An overcenter mechanism according to claim 6, wherein the surface on the external member is flat and the surface on the link member is a curved surface having less curvature than a circular arc described about said pivot pin.

8. An overcenter mechanism according to claim 7, wherein said curved surface is substantial a circular are described about the second end of the other of said link members.

9, An overcenter mechanism according to claim 6, wherein the surface on the external member is a curved surface comprising substantially a circular are described about the second end of the other of said link members, and wherein the surface on said one link member is a curved surface comprising substantially a circular are described about said pivot pin.

10. An overcenter mechanism according to claim 4, wherein said interengaging means for interengaging said first ends of the link members comprises a pivot pin, and wherein said cooperating surfaces comprise a pair of curved surfaces formed each on a respective one of said second ends of said link members and each comprising substantially a circular are described about said pivot pin, said curved surfaces cooperating with flat surfaces formed on the respective 

1. In an overcenter mechanism comprising: a. first and second link members each having two ends; b. interengaging means pivotally interengaging first ends of said link members together; c. a pair of external members; d. and means engaging the second ends of said link members each with a respective one of said external members; e. said interengaging means being located between said external members for spreading of said external members apart, against a first force urging them together, by a second force acting on said link members in the vicinity of said interengaging means in a first direction transverse to said first force to urge said link members forwardly into a central position in which said interengaging means lies on a straight line between the points of engagement of said second ends with said external members, from a first lateral position in which said interengaging means is displaced to one side of said central position, or by a third force acting on said link members in a second direction opposite to said first direction to urge said link members rearwardly into said central position from a second lateral position displaced to the other side of said central position by an amount equal to the displacement of said first lateral position from said central position; the improvement wherein: f. at least one end of one of said link members and the member portion engaged thereby comprise a pair of cooperating surfaces so arranged that one surface rolls on the other surface as the mechanism is moved from either of said first and second lateral positions to said central position; and g. said surfaces being so shaped that said third force is different from said second force.
 2. An overcenter mechanism according to claim 1, wherein said surfaces are so shaped that said third force is less than said second force.
 3. An overcenter mechanism according to claim 1, wherein said surfaces are so shaped that said third force is substantially zero.
 4. An overcenter mechanism according to claim 1, wherein said surfaces are so shaped that said third force is a minor proportion of said second force.
 5. An overcenter mechanism according to claim 1, wherein said surfaces are so shaped that said third force is not more than approximately 20 percent of said second force.
 6. An overcenter mechanism according to claim 4, wherein said interengaging means for interengaging said first ends of the link members comprises a pivot pin, and wherein said cooperating surfaces are formed respectively on a second end of one of said link members, and on one of said external members.
 7. An overcenter mechanism according to claim 6, wherein the surface on the external member is flat and the surface on the link member is a curved surface having less curvature than a circular arc described about said pivot pin.
 8. An overcenter mechanism according to claim 7, wherein said curved surface is substantial a circUlar arc described about the second end of the other of said link members.
 9. An overcenter mechanism according to claim 6, wherein the surface on the external member is a curved surface comprising substantially a circular arc described about the second end of the other of said link members, and wherein the surface on said one link member is a curved surface comprising substantially a circular arc described about said pivot pin.
 10. An overcenter mechanism according to claim 4, wherein said interengaging means for interengaging said first ends of the link members comprises a pivot pin, and wherein said cooperating surfaces comprise a pair of curved surfaces formed each on a respective one of said second ends of said link members and each comprising substantially a circular arc described about said pivot pin, said curved surfaces cooperating with flat surfaces formed on the respective external members.
 11. An overcenter mechanism according to claim 4, wherein said means engaging said second ends of the link members with a respective one of said external members comprise a pair of pivot pins, and wherein said interengaging means comprise a pair of cooperating curved surfaces formed each on a respective one of said first ends of said link members and each comprising substantially a circular arc described about the said pivot pin engaging the second end of the respective link member. 